Implantable medical device fixation

ABSTRACT

Various fixation techniques for implantable medical device (IMDs) are described. In one example, an assembly comprises an IMD; and a set of active fixation tines attached to the IMD. The active fixation tines in the set are deployable from a spring-loaded position in which distal ends of the active fixation tines point away from the IMD to a hooked position in which the active fixation tines bend back towards the IMD. The active fixation tines are configured to secure the IMD to a patient tissue when deployed while the distal ends of the active fixation tines are positioned adjacent to the patient tissue.

This application is a continuation of U.S. patent application Ser. No.14/193,306, filed on Feb. 28, 2014, which is a continuation of U.S.patent application Ser. No. 13/096,881, filed on Apr. 28, 2011, nowissued as U.S. Pat. No. 9,775,982, which claims the benefit of, andpriority to, U.S. Provisional Application Ser. No. 61/428,067, filed onDec. 29, 2010, the entire content of each of the applications identifiedabove being incorporated by reference herein.

TECHNICAL FIELD

This disclosure relates to fixation techniques for implantable medicaldevices.

BACKGROUND

Medical devices such as electrical stimulators, leads, and electrodesare implanted to deliver therapy to one or more target sites within thebody of a patient. To ensure reliable electrical contact between theelectrodes and the target site, fixation of the device, lead, orelectrodes is desirable.

A variety of medical devices for delivering a therapy and/or monitoringa physiological condition have been used clinically or proposed forclinical use in patients. Examples include medical devices that delivertherapy to and/or monitor conditions associated with the heart, muscle,nerve, brain, stomach or other organs or tissue. Some therapies includethe delivery of electrical signals, e.g., stimulation, to such organs ortissues. Some medical devices may employ one or more elongatedelectrical leads carrying electrodes for the delivery of therapeuticelectrical signals to such organs or tissues, electrodes for sensingintrinsic electrical signals within the patient, which may be generatedby such organs or tissue, and/or other sensors for sensing physiologicalparameters of a patient.

Medical leads may be configured to allow electrodes or other sensors tobe positioned at desired locations for delivery of therapeuticelectrical signals or sensing. For example, electrodes or sensors may becarried at a distal portion of a lead. A proximal portion of the leadmay be coupled to a medical device housing, which may contain circuitrysuch as signal generation and/or sensing circuitry. In some cases, themedical leads and the medical device housing are implantable within thepatient. Medical devices with a housing configured for implantationwithin the patient may be referred to as implantable medical devices(IMDs).

Implantable cardiac pacemakers or cardioverter-defibrillators, forexample, provide therapeutic electrical signals to the heart, e.g., viaelectrodes carried by one or more implantable medical leads. Thetherapeutic electrical signals may include pulses for pacing, or shocksfor cardioversion or defibrillation. In some cases, a medical device maysense intrinsic depolarizations of the heart, and control delivery oftherapeutic signals to the heart based on the sensed depolarizations.Upon detection of an abnormal rhythm, such as bradycardia, tachycardiaor fibrillation, an appropriate therapeutic electrical signal or signalsmay be delivered to restore or maintain a more normal rhythm. Forexample, in some cases, an IMD may deliver pacing stimulation to theheart of the patient upon detecting tachycardia or bradycardia, anddeliver cardioversion or defibrillation shocks to the heart upondetecting fibrillation.

Leadless IMDs may also be used to deliver therapy to a patient, and/orsense physiological parameters of a patient. In some examples, aleadless IMD may include one or more electrodes on its outer housing todeliver therapeutic electrical signals to patient, and/or senseintrinsic electrical signals of patient. For example, leadless cardiacdevices, such as leadless pacemakers, may also be used to senseintrinsic depolarizations and/or other physiological parameters of theheart and/or deliver therapeutic electrical signals to the heart. Aleadless cardiac device may include one or more electrodes on its outerhousing to deliver therapeutic electrical signals and/or sense intrinsicdepolarizations of the heart. Leadless cardiac devices may be positionedwithin or outside of the heart and, in some examples, may be anchored toa wall of the heart via a fixation mechanism.

SUMMARY

In general, this disclosure describes remotely-deployable activefixation tines for fixating IMDs or their components, such as leads, topatient tissues. As referred to herein an “IMD component,” may be anentire IMD or an individual component thereof. Examples of IMDs that maybe fixated to patient tissues with remotely-deployable active fixationtines according to this disclosure include leadless pacemakers andleadless sensing devices.

Active fixation tines disclosed herein may be deployed from the distalend of a catheter located at a desired implantation location for the IMDor its component. As further disclosed herein, active fixation tinesprovide a deployment energy sufficient to permeate a desired patienttissue and secure an IMD or its component to the patient tissue withouttearing the patient tissue. This disclosure includes active fixationtines that allow for removal from a patient tissue followed byredeployment, e.g., to adjust the position of the IMD relative to thepatient tissue. As different patient tissues have different physical andmechanical characteristics, the design of active fixation tines may becoordinated with patient tissue located at a selected fixation sitewithin a patient. Multiple designs may be used to optimize fixation fora variety of patient tissues.

In one example, the disclosure is directed to an assembly comprising: animplantable medical device; and a set of active fixation tines attachedto the implantable medical device. The active fixation tines in the setare deployable from a spring-loaded position in which distal ends of theactive fixation tines point away from the implantable medical device toa hooked position in which the active fixation tines bend back towardsthe implantable medical device. The active fixation tines are configuredto secure the implantable medical device to a patient tissue whendeployed while the distal ends of the active fixation tines arepositioned adjacent to the patient tissue.

In another example, the disclosure is directed to a kit for implantingan implantable medical device within a patient, the kit comprising: theimplantable medical device; a set of active fixation tines attached tothe implantable medical device. The active fixation tines in the set aredeployable from a spring-loaded position in which distal ends of theactive fixation tines point away from the implantable medical device toa hooked position in which the active fixation tines bend back towardsthe implantable medical device. The active fixation tines are configuredto secure the implantable medical device to a patient tissue whendeployed while the distal ends of the active fixation tines arepositioned adjacent to the patient tissue. The kit further comprises acatheter forming a lumen sized to receive the implantable medical deviceand hold the active fixation tines in the spring-loaded position,wherein the lumen includes an aperture that is adjacent to the distalend of the catheter; and a deployment element configured to initiatedeployment of the active fixation tines while the implantable medicaldevice is positioned within the lumen of the catheter. Deployment of theactive fixation tines while the implantable medical device is positionedwithin the lumen of the catheter causes the active fixation tines topull the implantable medical device out of the lumen via the aperturethat is adjacent to the distal end of the catheter.

In another example, the disclosure is directed to a method comprising:obtaining an assembly comprising an implantable medical device and a setof active fixation tines attached to the implantable medical device;positioning the distal ends of the active fixation tines adjacent to apatient tissue; and deploying the active fixation tines from aspring-loaded position in which distal ends of the active fixation tinespoint away from the implantable medical device to a hooked position inwhich the active fixation tines bend back towards the implantablemedical device to secure the implantable medical device to the patienttissue.

The details of one or more aspects of the disclosure are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the disclosure will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example therapy systemcomprising a leadless IMD that may be used to monitor one or morephysiological parameters of a patient and/or provide therapy to theheart of a patient.

FIG. 2 is a conceptual diagram illustrating another example therapysystem comprising an IMD coupled to a plurality of leads that may beused to monitor one or more physiological parameters of a patient and/orprovide therapy to the heart of a patient.

FIGS. 3A-3B illustrate the leadless IMD of FIG. 1 in further detail.

FIGS. 4A-4B illustrate an assembly including the leadless IMD of FIG. 1and a catheter configured to deploy the leadless IMD of FIG. 1

FIGS. 5A-5H illustrate techniques for securing the leadless IMD of FIG.1 to a patient tissue using the catheter of FIG. 4A-4B.

FIGS. 6A-6B illustrate an active fixation tine showing measurements usedto calculate performance characteristics of the active fixation tine.

FIGS. 7A-7D illustrate exemplary tine profiles.

FIG. 8 is a functional block diagram illustrating an exampleconfiguration of an IMD.

FIG. 9 is a block diagram of an example external programmer thatfacilitates user communication with an IMD.

FIG. 10 is a flowchart illustrating techniques for implanting animplantable medical device within a patient.

DETAILED DESCRIPTION

Active fixation tines disclosed herein may be useful to secure animplantable medical device (IMD) including any components thereof, suchas a medical lead, to a patient tissue during minimally invasivesurgery. Minimally invasive surgery, such as percutaneous surgery,permits IMD implantation with less pain and recovery time than opensurgery. However, minimally invasive surgery tends to be morecomplicated than open surgery. For example, forming device fixationrequires a surgeon to manipulate instruments remotely, e.g., within theconfines of an intravascular catheter. With techniques for remotedeployment and fixation of IMDs it can be difficult to ensure adequatefixation while minimizing tissue damage. The active fixation tinesdisclosed are suitable for securing an IMD to a patient tissue. Inaddition, active fixation tines disclosed herein also allow for simpleremoval from a patient tissue without tearing the patient tissuefollowed by redeployment, e.g., to adjust the position of the IMD afterfirst securing the IMD to the patient tissue.

In one example, active fixation tines disclosed herein may be deployedfrom the distal end of a catheter positioned by a clinician at a desiredimplantation location for the IMD. As further disclosed herein, activefixation tines provide a deployment energy sufficient to permeate adesired patient tissue and secure an IMD to the patient tissue withouttearing the patient tissue. As different patient tissues have differentphysical and mechanical characteristics, the design of active fixationtines may be configured according to the properties of the patienttissue located at a selected fixation site within a patient. Multipledesigns may be made for a variety of patient tissues, and available forselection based on the patient tissue at the fixation site.

Although various examples are described with respect to cardiac leadsand leadless IMD, the disclosed active fixation tines may be useful forfixation of a variety of implantable medical devices in a variety ofanatomical locations, and fixation of cardiac leads and leadless IMD isdescribed for purposes of illustration. The described techniques can bereadily applied securing catheters and other medical leads, e.g., forneurostimulation. As examples, medical leads with active fixation tinesmay be used for cardiac stimulation, gastric stimulation, functionalelectrical stimulation, peripheral nerve stimulation, spinal cordstimulation, pelvic nerve stimulation, deep brain stimulation, orsubcutaneous neurological stimulation as well as other forms ofstimulation. In addition, described techniques can be readily applied toIMDs including sensors, including leadless IMDs and IMDs with medicalleads. As examples, IMDs including sensors and active fixation tines mayinclude one or more of the following sensors: a pressure sensor, anelectrocardiogram sensor, an oxygen sensor (for tissue oxygen or bloodoxygen sensing), an accelerometer, a glucose sensor, a potassium sensor,a thermometer and/or other sensors.

FIG. 1 is a conceptual diagram illustrating an example therapy system10A that may be used to monitor one or more physiological parameters ofpatient 14 and/or to provide therapy to heart 12 of patient 14. Therapysystem 10A includes IMD 16A, which is coupled to programmer 24. IMD 16Amay be an implantable leadless pacemaker that provides electricalsignals to heart 12 via one or more electrodes (not shown in FIG. 1) onits outer housing. Additionally or alternatively, IMD 16A may senseelectrical signals attendant to the depolarization and repolarization ofheart 12 via electrodes on its outer housing. In some examples, IMD 16Aprovides pacing pulses to heart 12 based on the electrical signalssensed within heart 12.

IMD 16A includes a set of active fixation tines to secure IMD 16A to apatient tissue. In the example of FIG. 1, IMD 16A is positioned whollywithin heart 12 proximate to an inner wall of right ventricle 28 toprovide right ventricular (RV) pacing. Although IMD 16A is shown withinheart 12 and proximate to an inner wall of right ventricle 28 in theexample of FIG. 1, IMD 16A may be positioned at any other locationoutside or within heart 12. For example, IMD 16A may be positionedoutside or within right atrium 26, left atrium 36, and/or left ventricle32, e.g., to provide right atrial, left atrial, and left ventricularpacing, respectively.

Depending on the location of implant, IMD 16A may include otherstimulation functionalities. For example, IMD 16A may provideatrioventricular nodal stimulation, fat pad stimulation, vagalstimulation, or other types of neurostimulation. In other examples, IMD16A may be a monitor that senses one or more parameters of heart 12 andmay not provide any stimulation functionality. In some examples, system10A may include a plurality of leadless IMDs 16A, e.g., to providestimulation and/or sensing at a variety of locations.

As discussed in greater detail with respect to FIGS. 3A-5H, IMD 16Aincludes a set of active fixation tines. The active fixation tines inthe set are deployable from a spring-loaded position in which distalends of the active fixation tines point away from the IMD to a hookedposition in which the active fixation tines bend back towards the IMD.The active fixation tines allow IMD 16A to be removed from a patienttissue followed by redeployment, e.g., to adjust the position of IMD 16Arelative to the patient tissue. For example, a clinician implanting IMD16A may reposition IMD 16A during an implantation procedure if testingof IMD 16A indicates a poor electrode-tissue connection.

FIG. 1 further depicts programmer 24 in wireless communication with IMD16A. In some examples, programmer 24 comprises a handheld computingdevice, computer workstation, or networked computing device. Programmer24, shown and described in more detail below with respect to FIG. 9,includes a user interface that presents information to and receivesinput from a user. It should be noted that the user may also interactwith programmer 24 remotely via a networked computing device.

A user, such as a physician, technician, surgeon, electrophysiologist,other clinician, or patient, interacts with programmer 24 to communicatewith IMD 16A. For example, the user may interact with programmer 24 toretrieve physiological or diagnostic information from IMD 16A. A usermay also interact with programmer 24 to program IMD 16A, e.g., selectvalues for operational parameters of the IMD 16A. For example, the usermay use programmer 24 to retrieve information from IMD 16A regarding therhythm of heart 12, trends therein over time, or arrhythmic episodes.

As an example, the user may use programmer 24 to retrieve informationfrom IMD 16A regarding other sensed physiological parameters of patient14 or information derived from sensed physiological parameters, such asintracardiac or intravascular pressure, activity, posture, tissue oxygenlevels, blood oxygen levels, respiration, tissue perfusion, heartsounds, cardiac electrogram (EGM), intracardiac impedance, or thoracicimpedance. In some examples, the user may use programmer 24 to retrieveinformation from IMD 16A regarding the performance or integrity of IMD16A or other components of system 10A, or a power source of IMD 16A. Asanother example, the user may interact with programmer 24 to program,e.g., select parameters for, therapies provided by IMD 16A, such aspacing and, optionally, neurostimulation.

IMD 16A and programmer 24 may communicate via wireless communicationusing any techniques known in the art. Examples of communicationtechniques may include, for example, low frequency or radiofrequency(RF) telemetry, but other techniques are also contemplated. In someexamples, programmer 24 may include a programming head that may beplaced proximate to the patient's body near the IMD 16A implant site inorder to improve the quality or security of communication between IMD16A and programmer 24.

FIG. 2 is a conceptual diagram illustrating another example therapysystem 10B that may be used to monitor one or more physiologicalparameters of patient 14 and/or to provide therapy to heart 12 ofpatient 14. Therapy system 10B includes IMD 16B, which is coupled tomedical leads 18, 20, and 22, and programmer 24. As referred to herein,each of IMD 16B and medical leads 18, 20 and 22 may be referred togenerally as an IMD. In one example, IMD 16B may be an implantablepacemaker that provides electrical signals to heart 12 via electrodescoupled to one or more of leads 18, 20, and 22. IMD 16B is one exampleof an electrical stimulation generator, and is configured attached tothe proximal end of medical leads 18, 20, and 22. In other examples, inaddition to or alternatively to pacing therapy, IMD 16B may deliverneurostimulation signals. In some examples, IMD 16B may also includecardioversion and/or defibrillation functionalities. In other examples,IMD 16B may not provide any stimulation functionalities and, instead,may be a dedicated monitoring device. Patient 14 is ordinarily, but notnecessarily, a human patient.

Medical leads 18, 20, 22 extend into the heart 12 of patient 14 to senseelectrical activity of heart 12 and/or deliver electrical stimulation toheart 12. In the example shown in FIG. 2, right ventricular (RV) lead 18extends through one or more veins (not shown), the superior vena cava(not shown), right atrium 26, and into right ventricle 28. RV lead 18may be used to deliver RV pacing to heart 12. Left ventricular (LV) lead20 extends through one or more veins, the vena cava, right atrium 26,and into the coronary sinus 30 to a region adjacent to the free wall ofleft ventricle 32 of heart 12. LV lead 20 may be used to deliver LVpacing to heart 12. Right atrial (RA) lead 22 extends through one ormore veins and the vena cava, and into the right atrium 26 of heart 12.RA lead 22 may be used to deliver RA pacing to heart 12.

In some examples, system 10B may additionally or alternatively includeone or more leads or lead segments (not shown in FIG. 2) that deploy oneor more electrodes within the vena cava or other vein, or within or nearthe aorta. Furthermore, in another example, system 10B may additionallyor alternatively include one or more additional intravenous orextravascular leads or lead segments that deploy one or more electrodesepicardially, e.g., near an epicardial fat pad, or proximate to thevagus nerve. In other examples, system 10B need not include one ofventricular leads 18 and 20.

One or more of medical leads 18, 20, 22 may include a set of activefixation tines to secure a distal end of the medical lead to a patienttissue. The inclusion of active fixation tines for each medical leads18, 20, 22 is merely exemplary. One or more of medical leads 18, 20, 22could be secured by alternative techniques. For example, even thougheach of medical leads 18, 20 and 22 is shown with a set of activefixation tines to secure a distal end of the medical lead, LV lead 20,which extends through one or more veins and the vena cava and into theright atrium 26 of heart 12, may instead be fixed using passivefixation.

The active fixation tines in set active fixation tines attached to amedical lead are deployable from a spring-loaded position in whichdistal ends of the active fixation tines point away from the IMD to ahooked position in which the active fixation tines bend back towards theIMD. The active fixation tines allow the distal end of the medical leadbe removed from a patient tissue followed by redeployment, e.g., toadjust the position of the distal end of the medical lead relative tothe patient tissue. For example, a clinician implanting IMD 16B mayreposition the distal end of a medical lead during an implantationprocedure if testing of IMD 16B indicates a poor electrode-tissueconnection.

IMD 16B may sense electrical signals attendant to the depolarization andrepolarization of heart 12 via electrodes (described in further detailwith respect to FIG. 4) coupled to at least one of the leads 18, 20, 22.In some examples, IMD 16B provides pacing pulses to heart 12 based onthe electrical signals sensed within heart 12. The configurations ofelectrodes used by IMD 16B for sensing and pacing may be unipolar orbipolar.

IMD 16B may also provide neurostimulation therapy, defibrillationtherapy and/or cardioversion therapy via electrodes located on at leastone of the leads 18, 20, 22. For example, IMD 16B may deliverdefibrillation therapy to heart 12 in the form of electrical pulses upondetecting ventricular fibrillation of ventricles 28 and 32. In someexamples, IMD 16B may be programmed to deliver a progression oftherapies, e.g., pulses with increasing energy levels, until afibrillation of heart 12 is stopped. As another example, IMD 16B maydeliver cardioversion or anti-tachycardia pacing (ATP) in response todetecting ventricular tachycardia, such as tachycardia of ventricles 28and 32.

As described above with respect to IMD 16A of FIG. 1, programmer 24 mayalso be used to communicate with IMD 16B. In addition to the functionsdescribed with respect to IMD 16A of FIG. 1, a user may use programmer24 to retrieve information from IMD 16B regarding the performance orintegrity of leads 18, 20 and 22 and may interact with programmer 24 toprogram, e.g., select parameters for, any additional therapies providedby IMD 16B, such as cardioversion and/or defibrillation.

Leads 18, 20, 22 may be electrically coupled to a signal generator and asensing module of IMD 16B via connector block 34. In some examples,proximal ends of leads 18, 20, 22 may include electrical contacts thatelectrically couple to respective electrical contacts within connectorblock 34 of IMD 16B. In some examples, a single connector, e.g., an IS-4or DF-4 connector, may connect multiple electrical contacts to connectorblock 34. In addition, in some examples, leads 18, 20, 22 may bemechanically coupled to connector block 34 with the aid of set screws,connection pins, snap connectors, or another suitable mechanicalcoupling mechanism.

The configuration of system 10B illustrated in FIG. 2 is merely oneexample. In other examples, a system may include epicardial leads and/orpatch electrodes instead of or in addition to the transvenous leads 18,20, 22 illustrated in FIG. 2. Further, IMD 16B need not be implantedwithin patient 14. In examples in which IMD 16B is not implanted inpatient 14, IMD 16B may deliver defibrillation pulses and othertherapies to heart 12 via percutaneous leads that extend through theskin of patient 14 to a variety of positions within or outside of heart12. For each of these examples, any number of the medical leads mayinclude a set of active fixation tines on a distal end of the medicallead in accordance with the techniques described herein.

In addition, in other examples, a system may include any suitable numberof leads coupled to IMD 16B, and each of the leads may extend to anylocation within or proximate to heart 12. For example, other examples ofsystems may include three transvenous leads located as illustrated inFIG. 2, and an additional lead located within or proximate to leftatrium 36. Other examples of systems may include a single lead thatextends from IMD 16B into right atrium 26 or right ventricle 28, or twoleads that extend into a respective one of the right ventricle 28 andright atrium 26. Any electrodes located on these additional leads may beused in sensing and/or stimulation configurations. In each of theseexamples, any number of the medical leads may include a set of activefixation tines on a distal end of the medical lead in accordance withthe techniques described herein.

FIGS. 3A-3B illustrate leadless IMD 16A of FIG. 1 in further detail. Inthe example of FIGS. 3A and 3B, leadless IMD 16A includes tine fixationsubassembly 100 and electronic subassembly 150. Tine fixationsubassembly 100 is configured to anchor leadless IMD 16A to a patienttissue, such as a wall of heart 12.

Electronic subassembly 150 includes control electronics 152, whichcontrols the sensing and/or therapy functions of IMD 16A, and battery160, which powers control electronics 152. As one example, controlelectronics 152 may include sensing circuitry, a stimulation generatorand a telemetry module. As one example, battery 160 may comprisefeatures of the batteries disclosed in U.S. patent application Ser. No.12/696,890, titled IMPLANTABLE MEDICAL DEVICE BATTERY and filed Jan. 29,2010, the entire contents of which are incorporated by reference herein.

The housings of control electronics 152 and battery 160 are formed froma biocompatible material, such as a stainless steel or titanium alloy.In some examples, the housings of control electronics 152 and battery160 may include an insulating coating. Examples of insulating coatingsinclude parylene, urethane, PEEK, or polyimide among others. Electronicsubassembly 150 further includes anode 162, which may include a lowpolarizing coating, such as titanium nitride, iridium oxide, rutheniumoxide among others. The entirety of the housings of control electronics152 and battery 160 are electrically connected to one another, but onlyanode 162 is uninsulated. In other examples, the entirety of the housingof battery 160 or the entirety of the housing of electronic subassembly150 may function as an anode instead of providing a localized anode suchas anode 162. Alternatively, anode 162 may be electrically isolated fromthe other portions of the housings of control electronics 152 andbattery 160.

Delivery tool interface 158 is located at the proximal end of electronicsubassembly 150. Delivery tool interface 158 is configured to connect toa delivery device, such as catheter 200 (FIG. 5A) used to position IMD16A during an implantation procedure. Tine fixation subassemblyinterface 153 and feedthrough pin 154 are located at the distal end ofelectronic subassembly 150. Tine fixation subassembly interface 153includes three tabs that interlock with tine fixation subassembly 100.

As best illustrated in FIG. 3B, tine fixation subassembly 100 includesfixation element 102, header body 112, header cap 114, locking tab 120,electrode 122, monolithic controlled release device (MCRD) 124 andfiller cap 126. Fixation element 102 includes a set of four activefixation tines 103 that are deployable from a spring-loaded position inwhich distal ends of active fixation tines 103 point away fromelectronic subassembly 150 to a hooked position in which active fixationtines 103 bend back towards electronic subassembly 150. For example,active fixation tines 103 are shown in the hooked position in FIG. 3A.As discussed in further detail with respect to FIGS. 4A-5H, activefixation tines 103 are configured to secure IMD 16A to a patient tissue,e.g., a tissue inside the heart or outside the heart, when deployedwhile the distal ends of active fixation tines 103 are positionedadjacent to the patient tissue. In different examples, active fixationtines 103 may be positioned adjacent to patient tissue such that distalends 109 penetrate the patient tissue prior to deployment, positionedadjacent to patient tissue such that distal ends 109 contact but do notpenetrate the patient tissue prior to deployment or positioned adjacentto patient tissue such that distal ends 109 are near to but do notcontact or penetrate the patient tissue prior to deployment.

Fixation element 102 may be fabricated of a shape memory material, whichallows active fixation tines 103 to bend elastically from the hookedposition to the spring-loaded position. As an example, the shape memorymaterial may be shape memory alloy such as Nitinol. In one example,fixation element 102 including active fixation tines 103 and base 111,may be manufactured by cutting fixation element 102 as a unitarycomponent from a hollow tube of Nitinol, bending the cut tube to formthe hooked position shape of active fixation tines 103 and heat-treatingfixation element 102 while holding active fixation tines 103 in thehooked position. Sharp edges of fixation element 102 may be rounded offto improve fatigue loading and reduce tearing of patient tissue duringdeployment and retraction of active fixation tines 103.

In some examples, all or a portion of fixation element 102, such asactive fixation tines 103, may include one or more coatings. Forexample, fixation element 102 may include a radiopaque coating toprovide visibility during fluoroscopy. In one such example, fixationelement 102 may include one or more radiopaque markers. As anotherexample, fixation element 102 may be coated with a tissue growthpromoter or a tissue growth inhibitor. A tissue growth promoter may beuseful to increase the holding force of active fixation tines 103,whereas a tissue growth inhibitor may be useful to facilitate removal ofIMD 16A during an explantation procedure, which may occur many yearsafter the implantation of IMD 16A.

During assembly of IMD 16A, prior to being mounted to electronicsubassembly 150, fixation element 102 may be mounted in a headerincluding header body 112 and header cap 114. For example, fixationelement 102 may be mounted such that one tine extends though each ofholes 113 in header body 112. Then header cap 114 is positioned overbase 111 of fixation element 102 and secured to header body 112. As anexample, header body 112 and header cap 114 may be fabricated of abiocompatible polymer such as polyether ether ketone (PEEK). Header body112 and header cap 114 may function to electrically isolate fixationelement 102 from electronic subassembly 150 and feedthrough pin 154. Inother examples, fixation element 102 itself may be used as an electrodefor stimulation and/or sensing a physiological condition of a patientand may electrically connect to control electronics 152.

During assembly of IMD 16A, once fixation element 102 is assembled withheader body 112 and header cap 114, fixation element 102, header body112 and header cap 114 are mounted to the tabs of tine fixationsubassembly interface 153 on electronic subassembly 150 by positioningheader body 112 over the tabs of tine fixation subassembly interface 153and rotating header body 112 to interlock header body 112 with the tabsof tine fixation subassembly interface 153. Feedthrough pin 154 extendsthrough the center of header body 112 once header body 112 is secured totine fixation subassembly interface 153.

During assembly of IMD 16A, after header body 112 is secured to tinefixation subassembly interface 153, locking tab 120 is positioned overfeedthrough pin 154. As an example, locking tab 120 may be fabricated ofa silicone material. Next, electrode 122 is positioned over locking tab120 and feedthrough pin 154, and then mechanically and electricallyconnected to feedthrough pin 154, e.g., using a laser weld. As anexample, electrode 122 may comprise a biocompatible metal, such as aniridium alloy or a platinum alloy.

MCRD 124 is located within recess 123 of electrode 122. In theillustrated example, MCRD 124 takes the form of a cylindrical plug. Inother examples, an MCRD band may positioned around the outside of theelectrode rather than configured as a cylindrical plug. MCRD 124 may befabricated of a silicone based polymer, or other polymers. MCRD 124 mayincorporate an anti-inflammatory drug, which may be, for example, thesodium salt of dexamethasone phosphate. Because MCRD 124 is retainedwithin recess 123 of electrode 122, migration of the drug contained inMCRD 124 is limited to the tissue in contact with the distal end ofelectrode 122. Filler cap 126 is positioned over electrode 122. As anexample, filler cap 126 may be fabricated of a silicone material andpositioned over both electrode 122 and locking tab 120 during assemblyof IMD 16A.

As different patient tissues have different physical and mechanicalcharacteristics, active fixation tines 103 may be specifically designedto perform with patient tissues having specific characteristics. Forexample, active fixation tines 103 may be designed to provide a selectedfixation force, designed to penetrate to a particular depth of a patienttissue, designed to penetrate to a particular layer of patient tissue(as different tissue layers may have different mechanical properties)and/or designed to facilitate removal and redeployment from the patienttissue without tearing the patient tissue, either on deployment orremoval. Multiple designs of active fixation tine 103 may be used tooptimize fixation for a variety of patient tissues. The design of activefixation tine 103 is discussed in further detail with respect to FIGS.6A-6B. In addition, the specific design of tine fixation subassembly 100is not germane to the operation of active fixation tines 103, and avariety of techniques may be used to attach a set of active fixationtines to an IMD.

FIG. 4A illustrates assembly 180, which includes leadless IMD 16A andcatheter 200, which is configured to remotely deploy IMD 16A. Catheter200 may be a steerable catheter or be configured to traverse aguidewire. In any case, catheter 200 may be directed within a bodylumen, such as a vascular structure to a target site in order tofacilitate remote positioning and deployment of IMD 16A. In particular,catheter 200 forms lumen 201, which is sized to receive IMD 16A at thedistal end of catheter 200. For example, the inner diameter of lumen 201at the distal end of catheter 200 may be about the same size as theouter diameter of IMD 16A. When IMD 16A is positioned within lumen 201at the distal end of catheter 200, lumen 201 holds active fixation tines103 in the spring-loaded position shown in FIG. 4A. In the spring-loadedposition, active fixation tines 103 store enough potential energy tosecure IMD 16A to a patient tissue upon deployment.

Lumen 201 includes aperture 221, which is positioned at the distal endof catheter 200. Aperture 221 facilitates deployment of IMD 16A.Deployment element 210 is positioned proximate to IMD 16A in lumen 201.Deployment element 210 configured to initiate deployment of activefixation tines 103. More particularly, a clinician may remotely deployIMD 16A by pressing plunger 212, which is located at the proximal end ofcatheter 200. Plunger 212 connects directly to deployment element 210,e.g., with a wire or other stiff element running through catheter 200,such that pressing on plunger 212 moves deployment element 210 distallywithin lumen 201. As deployment element 210 moves distally within lumen201, deployment element 210 pushes IMD 16A distally within lumen 201 andtowards aperture 221. Once the distal ends 109 of active fixation tines103 reach aperture 221, active fixation tines 103 pull IMD 16A out oflumen 201 via aperture 221 as active fixation tines 103 move from aspring-loaded position to a hooked position to deploy IMD 16A. Thepotential energy released by active fixation tines 103 is sufficient topenetrate a patient tissue and secure IMD 16A to the patient tissue.

Tether 220 is attached to delivery tool interface 158 (not shown in FIG.4A) of IMD 16A and extends through catheter 200. Following deployment ofIMD 16A, a clinician may remotely pull IMD 16A back into lumen 201 bypulling on tether 220 at the proximal end of catheter 200. Pulling IMD16A back into lumen 201 returns active fixation tines 103 to thespring-loaded position from the hooked position. The proximal ends ofactive fixation tines 103 remain fixed to the housing of IMD 16A asactive fixation tines 103 move from the spring-loaded position to thehooked position and vice-versa. Active fixation tines 103 are configuredto facilitate releasing IMD 16A from patient tissue without tearing thetissue when IMD 16A is pulled back into lumen 201 by tether 220. Aclinician may redeploy IMD 16A with deployment element 210 by operatingplunger 212.

FIG. 4B is a sectional view of the distal end of assembly 180 in whichIMD 16A is positioned within lumen 201. Lumen 201 holds active fixationtines 103 in a spring-loaded position. Distal ends 109 of activefixation tines 103 are indicated in FIG. 4B. As shown in FIG. 4B, thefour active fixation tines 103 are positioned substantially equidistantfrom each other in a circular arrangement. As best seen in FIG. 3A,active fixation tines 103 are oriented outwardly relative to thecircular arrangement.

Positioning active fixation tines 103 substantially equidistant fromeach other in a circular arrangement creates opposing radial forces 222when active fixation tines 103 are deployed in unison. This allows thecombined forces of active fixation tines 103 acting on the distal end ofcatheter 200 to pull IMD 16A about perpendicularly out of aperture 221.When the active fixation tines are deployed while aperture 221 anddistal ends 109 of active fixation tines 103 are positioned adjacent toa patient tissue, the forces of active fixation tines 103 acting on thedistal end of catheter 200 combine to pull IMD 16A straight out fromaperture 221 and directly towards the patient tissue. While IMD 16Aincludes a set of four active fixation tines, a set of more or less thanfour active fixation tines may be used. For example, as few as twoactive fixation tines may provide opposing radial forces 222; however, aset of at least three active fixation tines may provide betterdirectional consistency in the deployment of an IMD such as IMD 16A.

Distal ends 109 of active fixation tines 103 include substantially flatouter surfaces to register active fixation tines 103 on the innersurface of lumen 201. The flat outer surfaces of active fixation tines103 help ensure that the interaction between active fixation tines 103and the inner surface of lumen 201 during deployment of IMD 16A providesopposing radial forces 222.

FIGS. 5A-5H illustrate example techniques for securing IMD 16A topatient tissue 300 using catheter 200. As an example, patient tissue 300may be a heart tissue, such as the inner wall of the right ventricle.For simplicity, a set of only two active fixation tines 103 are shown ineach of FIGS. 5A-5H; however, the described techniques for securing IMD16A to patient tissue 300 are equally applicable to IMDs including a setof more than two active fixation tines 103.

FIG. 5A illustrates IMD 16A within lumen 201 of catheter 200. Lumen 201holds active fixation tines 103 in a spring-loaded position in whichdistal ends 109 of active fixation tines 103 point away from IMD 16A.Aperture 221 is positioned adjacent patient tissue 300. The distal end202 of catheter 200 may not pressed forcefully into patient tissue 300,as pressing patient tissue 300 would alter the mechanicalcharacteristics of patient tissue 300. As active fixation tines 103 maybe designed accordingly to the mechanical characteristics of patienttissue 300, altering the mechanical characteristics of patient tissue300 may undesirably alter the interaction of active fixation tines 103and patient tissue 300 during deployment of active fixation tines 103.In other examples, it may be desirable to alter the mechanicalcharacteristics of patient tissue 300 for deployment, by significantlypressing on patient tissue 300 during deployment or by otherwisealtering the mechanical characteristics of patient tissue 300, toachieve a desired interaction (e.g., tissue permeation, fixation depth,etc.) between patient tissue 300 and active fixation tines 103 duringdeployment of active fixation tines 103.

FIG. 5B illustrates IMD 16A shortly after a clinician remotely activatedactive fixation tines 103 using deployment element 210 by pressing onplunger 212 (FIG. 4A). As the clinician pressed plunger 212, deploymentelement 210 pushed IMD 16A distally within lumen 201. Once the distalends 109 of active fixation tines 103 reached aperture 221, activefixation tines 103 began to pull IMD 16A out of lumen 201 via aperture221. Distal ends 109 of active fixation tines 103 then penetratedpatient tissue 300. FIG. 5B illustrates active fixation tines 103 in aposition after distal ends 109 of active fixation tines 103 penetratedpatient tissue 300 and shortly after beginning the transition from aspring-loaded position to a hooked position.

FIGS. 5B-5F illustrates active fixation tines 103 as they move from aspring-loaded position in which distal ends 109 of active fixation tines103 point away from IMD 16A to a hooked position in which distal ends109 of active fixation tines 103 bend back towards IMD 16A. FIGS. 5D-5Fillustrate active fixation tines 103 in hooked positions. In FIG. 5D,distal ends 109 of active fixation tines 103 remain embedded in patienttissue 300, whereas FIGS. 5E-5F illustrate distal ends 109 of activefixation tines 103 penetrating out of patient tissue 300.

As active fixation tines 103 move from a spring-loaded position to ahooked position, potential energy stored in active fixation tines 103 isreleased as IMD 16A is pulled from lumen 201 via aperture 221. Inaddition, active fixation tines 103 penetrate patient tissue 300 tosecure IMD 16A to patient tissue 300 such that electrode 123 (FIG. 5E)contacts patient tissue 300 within the center of the circulararrangement of active fixation tines 103. Active fixation tines 103provide a forward pressure of electrode 123 onto tissue 300 to assuregood electrode-tissue contact.

As active fixation tines 103 pull IMD 16A from lumen 201, tether 220,which is attached to delivery tool interface 158 of IMD 16A is exposed,e.g., as shown in FIG. 5E. Following deployment of IMD 16A, a clinicianmay remotely pull IMD 16A back into lumen 201 by pulling on tether 220at the proximal end of catheter 200. For example, the clinician mayperform a test of IMD 16A to evaluate a performance characteristic ofelectrode 123 while the IMD 16A is secured to patient tissue 300 asshown in FIG. 5E. If the test of IMD 16A indicates inadequateperformance, the clinician may decide to redeploy IMD 16A. Pulling IMD16A back into lumen 201 releases IMD 16A from patient tissue 300 andreturns IMD 16A to the position shown in FIG. 5A. From this position aclinician may reposition IMD 16A as desired and redeploy IMD 16A.

As shown in FIG. 5F, once IMD 16A is secured to patient tissue 300 inthe desired position, the clinician may release IMD 16A from tether 220.For example, the clinician may sever tether 220 at the proximal end ofcatheter 200 and remove tether 220 from delivery tool interface 158 bypulling on one of the severed ends of tether 220. As shown in FIG. 5G,once IMD 16A is released from tether 220, the clinician may removecatheter 200, leaving IMD 16A secured to patient tissue 300. As shown inFIG. 5H, active fixation tines 103 may continue to migrate to alower-potential energy hooked position over time. However, any of thehooked positions of active fixation tines 103 as shown in FIGS. 5D-5Gmay be sufficient to adequately secure IMD 16A to patient tissue 300.

While the techniques of FIGS. 5A-5H are illustrated with respect to IMD16A, the techniques may also be applied to a different IMD, such as amedical lead including a set of active fixation tines like medical leads18, 20, 22 of IMD 16B (FIG. 2). For example, such a medical lead mayextend through a catheter during an implantation procedure. As such,deploying a medical lead may not require a separate deployment elementwithin the catheter. Instead, simply pushing on the medical lead at theproximal end of the catheter may initiate deployment of a set of activefixation tines at the distal end of the medical lead by pushing theactive fixation tines attached to the distal end of the medical lead outof the distal end of the catheter. Similarly retracting a medical leadfor redeployment may not require a tether, but may instead simplyinvolve pulling on the medical lead at the proximal end of the catheter.

FIGS. 6A-6B illustrate one active fixation tine 103 and furtherillustrate measurements used to calculate performance characteristics ofactive fixation tine 103. In particular, FIG. 6A illustrates across-section of active fixation tine 103 with width 104 and thickness(T) 105. FIG. 6B illustrates a side-view of active fixation tine 103with tine length (L) 106, tine radius (r) 107 and tine angle 108.

The design of active fixation tine 103 is based on many criteria. As oneexample, an active fixation tine must penetrate a patient tissue whenextended in the spring-loaded position. To meet this criteria, length106 must be large enough to overcome the elasticity of the patienttissue such that distal end 109 of active fixation tine 103 permeatesthe patient tissue before active fixation tine 103 starts to bendsignificantly when deployed. For example, active fixation tine 103 willstart to bend significantly when deployed once the curved portion ofactive fixation tine 103 reaches aperture 221 in distal end 202 ofcatheter 200 (FIG. 4A).

If distal end 109 of active fixation tine 103 were pointed, this wouldreduce the insertion force; however, adding a sharp point to activefixation tine 103 may cause tearing of patient tissue during deploymentand removal of active fixation tine 103. For this reason, distal end 109of active fixation tine 103 may be rounded. As one example, tinethickness 105 may be between about 0.005 inches and about 0.010 inches.In a further example, tine thickness 105 may be between about 0.006inches and about 0.009 inches. In some examples, a tine may include aball on its distal end to further resist tearing of patient tissue. Onesuch example is shown in FIG. 7C.

As another example, the straight section providing length 106 of activefixation tine 103 must provide a column strength great enough to resistbuckling from the force of the patient tissue before distal end 109 ofactive fixation tine 103 permeates the patient tissue. Column strengthis dependent on length 106, width 104 and thickness 105, whereas theforce required to permeate a patient tissue is dependent on mechanicalproperties of the tissue and the cross-sectional area of distal end 109of active fixation tine 103. In addition, active fixation tine 103 maybe designed to buckle before penetrating a particular tissue layerdeeper than a targeted tissue layer. For example, when attaching toendocardial tissue, a tine may be designed to buckle before penetratingan epicardial layer of heart tissue to prevent penetrating an epicardiallayer of heart tissue during deployment.

As another example, a set of active fixation tines may be designed toprovide a selected holding force, which may also be referred to as thepull force required to remove a deployed set of active fixation tinesfrom patient tissue (or other material). As one example, a holding forceof between 1 and 5 newtons (N) or between 2 and 3 N may be suitable forsecuring IMD 16A within heart 12 (FIG. 1), while facilitating removal ofthe set of active fixation tines without tearing patient tissue.

Releasing an IMD from the tissue without tearing the tissue by pullingthe implantable medical device away from the tissue includes, pulling onthe implantable medical device to stretch the tissue until the tissuestiffness matches the tine straightening force, further pulling on theimplantable medical device until the tines straighten without tearingthe tissue, and continued pulling on the implantable medical device oncethe tines have straightened sufficiently to remove the tines from thepatient tissue. The pulling distance required to release the tines fromthe tissue is longer than the length og the tines because of theelasticity of the tissue. For an example, in an example wherein thetines 7 mm long, removing the tines from the tissue may require pullingthe IMD 12-20 mm away from the tissue.

Tine holding force may be considered the sum of tine straighteningforces (to move the active fixation tines from the hooked position tothe spring-loaded position) plus forces between the tine and the patienttissue, including frictional forces and forces that resist straighteningof the tine in the patient tissue. Using finite element analysis,validated by actual testing, the following transfer function of the pullforce required to remove a set of four active fixation tines deployed incardiac tissue was determined, wherein C₁:C₈ each represents a constantgreater than zero:Pull Force=−C ₁ +C ₂ *T−C ₃ *L+C ₄ *r−C ₅ *T*L−C ₆ *T*r−C ₇ *L*r+C ₈*T*L*r   (Equation 1)

A sensitivity analysis using a Pareto Chart of Effects on the importanceof the different factors of Equation 1 indicated that pull force is mostsensitive to tine thickness (59%), followed by tine radius (38%). Pullforce showed the least sensitivity to tine length (3%). In addition, theinteraction between thickness and radius was also important, whereas theother interactions were less significant.

In some examples, thickness greater than 0.009 inches or less than 0.003inches may not be able to produce a pull forces suitable for securingIMD 16A within heart 12 (FIG. 1). Of course, in other examples, e.g.,using a different selected holding forces, or assuming differentmaterial properties of active fixation tines 103 and/or of patienttissue tine thickness of greater than 0.009 inches or less than 0.003inches may be suitable.

One additional design factor is fatigue loading, e.g., fatigue loadingresulting from movement of a patient. For example, active fixation tines103 may be designed to secure IMD 16A to patient heart 12 for a periodof eighteen or more years. During that time, active fixation tines 103may experience about 600 million heart beats from heart 12. In addition,sharp corners are detrimental to withstanding fatigue loading; for thisreason, corners of active fixation tines 103 may be rounded, e.g., asbest shown in FIG. 6A.

FIGS. 7A-7D illustrate exemplary profiles of the distal ends ofdifferent active fixation tine designs. In particular, FIG. 7A,illustrates rectangular profile 410 that provides a consistent widththrough its distal end 412. A tine providing rectangular profile 410 mayalso provide a generally consistent thickness. As an example,rectangular profile 410 is consistent with the profile of activefixation tines 103.

FIG. 7B illustrates profile 420, which includes an increased width atits distal end 422. A tine providing profile 420 may also provide agenerally consistent thickness. Profile 420 may provide an increasedinsertion force and reduced column strength relative to tine profile410. In addition, a tine providing profile 420 may reduce tearing ofpatient tissue during insertion and removal relative to a tine providingtine profile 410.

FIG. 7C illustrates profile 430, with includes an enlarged distal tip432. Enlarged distal tip 432 is wider and thicker than the rest of atine providing profile 430. A tine including enlarged distal tip 432 mayreduce tearing of patient tissue during insertion and removal relativeto a tine providing tine profile 410.

FIG. 7D illustrates profile 440, which includes an increased width atits distal end 442. A tine providing profile 440 may also provide agenerally consistent thickness. Profile 440 also includes a series ofapertures 444. After implantation, a tine including apertures 444 mayprovide a significant increase in holding strength relative to tineproviding profile 410 as patient tissue grows around apertures 444. Inaddition, tine profile 440 may provide an increased insertion force andreduced column strength relative to tine profile 410.

FIG. 8 is a functional block diagram illustrating one exampleconfiguration of IMD 16A of FIGS. 1 and 3 or IMD 16B of FIG. 2 (referredto generally as IMD 16). In the example illustrated by FIG. 8, IMD 16includes a processor 80, memory 82, signal generator 84, electricalsensing module 86, telemetry module 88, and power source 89. Memory 82may include computer-readable instructions that, when executed byprocessor 80, cause IMD 16 and processor 80 to perform various functionsattributed to IMD 16 and processor 80 herein. Memory 82 may be acomputer-readable storage medium, including any volatile, non-volatile,magnetic, optical, or electrical media, such as a random access memory(RAM), read-only memory (ROM), non-volatile RAM (NVRAM),electrically-erasable programmable ROM (EEPROM), flash memory, or anyother digital or analog media.

Processor 80 may include any one or more of a microprocessor, acontroller, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field-programmable gate array (FPGA), orequivalent discrete or integrated logic circuitry. In some examples,processor 80 may include multiple components, such as any combination ofone or more microprocessors, one or more controllers, one or more DSPs,one or more ASICs, or one or more FPGAs, as well as other discrete orintegrated logic circuitry. The functions attributed to processor 80 inthis disclosure may be embodied as software, firmware, hardware or anycombination thereof. Processor 80 controls signal generator 84 todeliver stimulation therapy to heart 12 according to operationalparameters or programs, which may be stored in memory 82. For example,processor 80 may control signal generator 84 to deliver electricalpulses with the amplitudes, pulse widths, frequency, or electrodepolarities specified by the selected one or more therapy programs.

Signal generator 84, as well as electrical sensing module 86, iselectrically coupled to electrodes of IMD 16 and/or leads coupled to IMD16. In the example illustrated in FIG. 8, signal generator 84 isconfigured to generate and deliver electrical stimulation therapy toheart 12. For example, signal generator 84 may deliver pacing,cardioversion, defibrillation, and/or neurostimulation therapy via atleast a subset of the available electrodes. In some examples, signalgenerator 84 delivers one or more of these types of stimulation in theform of electrical pulses. In other examples, signal generator 84 maydeliver one or more of these types of stimulation in the form of othersignals, such as sine waves, square waves, or other substantiallycontinuous time signals.

Signal generator 84 may include a switch module and processor 80 may usethe switch module to select, e.g., via a data/address bus, which of theavailable electrodes are used to deliver stimulation signals, e.g.,pacing, cardioversion, defibrillation, and/or neurostimulation signals.The switch module may include a switch array, switch matrix,multiplexer, or any other type of switching device suitable toselectively couple a signal to selected electrodes.

Electrical sensing module 86 monitors signals from at least a subset ofthe available electrodes, e.g., to monitor electrical activity of heart12. Electrical sensing module 86 may also include a switch module toselect which of the available electrodes are used to sense the heartactivity. In some examples, processor 80 may select the electrodes thatfunction as sense electrodes, i.e., select the sensing configuration,via the switch module within electrical sensing module 86, e.g., byproviding signals via a data/address bus.

In some examples, electrical sensing module 86 includes multipledetection channels, each of which may comprise an amplifier. Eachsensing channel may detect electrical activity in respective chambers ofheart 12, and may be configured to detect either R-waves or P-waves. Insome examples, electrical sensing module 86 or processor 80 may includean analog-to-digital converter for digitizing the signal received from asensing channel for electrogram (EGM) signal processing by processor 80.In response to the signals from processor 80, the switch module withinelectrical sensing module 86 may couple the outputs from the selectedelectrodes to one of the detection channels or the analog-to-digitalconverter.

During pacing, escape interval counters maintained by processor 80 maybe reset upon sensing of R-waves and P-waves with respective detectionchannels of electrical sensing module 86. Signal generator 84 mayinclude pacer output circuits that are coupled, e.g., selectively by aswitching module, to any combination of the available electrodesappropriate for delivery of a bipolar or unipolar pacing pulse to one ormore of the chambers of heart 12. Processor 80 may control signalgenerator 84 to deliver a pacing pulse to a chamber upon expiration ofan escape interval. Processor 80 may reset the escape interval countersupon the generation of pacing pulses by signal generator 84, ordetection of an intrinsic depolarization in a chamber, and therebycontrol the basic timing of cardiac pacing functions. The escapeinterval counters may include P-P, V-V, RV-LV, A-V, A-RV, or A-LVinterval counters, as examples. The value of the count present in theescape interval counters when reset by sensed R-waves and P-waves may beused by processor 80 to measure the durations of R-R intervals, P-Pintervals, P-R intervals and R-P intervals. Processor 80 may use thecount in the interval counters to detect heart rate, such as an atrialrate or ventricular rate. In some examples, a leadless IMD with a set ofactive fixation tines may include one or more sensors in addition toelectrical sensing module 86. For example, a leadless IMD may include apressure sensor and/or an oxygen sensor (for tissue oxygen or bloodoxygen sensing).

Telemetry module 88 includes any suitable hardware, firmware, softwareor any combination thereof for communicating with another device, suchas programmer 24 (FIGS. 1 and 2). Under the control of processor 80,telemetry module 88 may receive downlink telemetry from and send uplinktelemetry to programmer 24 with the aid of an antenna, which may beinternal and/or external. Processor 80 may provide the data to beuplinked to programmer 24 and receive downlinked data from programmer 24via an address/data bus. In some examples, telemetry module 88 mayprovide received data to processor 80 via a multiplexer.

In some examples, processor 80 may transmit an alert that a mechanicalsensing channel has been activated to identify cardiac contractions toprogrammer 24 or another computing device via telemetry module 88 inresponse to a detected failure of an electrical sensing channel. Thealert may include an indication of the type of failure and/orconfirmation that the mechanical sensing channel is detecting cardiaccontractions. The alert may include a visual indication on a userinterface of programmer 24. Additionally or alternatively, the alert mayinclude vibration and/or audible notification. Processor 80 may alsotransmit data associated with the detected failure of the electricalsensing channel, e.g., the time that the failure occurred, impedancedata, and/or the inappropriate signal indicative of the detectedfailure.

FIG. 9 is a functional block diagram of an example configuration ofprogrammer 24. As shown in FIG. 9, programmer 24 includes processor 90,memory 92, user interface 94, telemetry module 96, and power source 98.Programmer 24 may be a dedicated hardware device with dedicated softwarefor programming of IMD 16. Alternatively, programmer 24 may be anoff-the-shelf computing device running an application that enablesprogrammer 24 to program IMD 16.

A user may use programmer 24 to select therapy programs (e.g., sets ofstimulation parameters), generate new therapy programs, or modifytherapy programs for IMD 16. The clinician may interact with programmer24 via user interface 94, which may include a display to present agraphical user interface to a user, and a keypad or another mechanismfor receiving input from a user.

Processor 90 can take the form of one or more microprocessors, DSPs,ASICs, FPGAs, programmable logic circuitry, or the like, and thefunctions attributed to processor 90 in this disclosure may be embodiedas hardware, firmware, software or any combination thereof. Memory 92may store instructions and information that cause processor 90 toprovide the functionality ascribed to programmer 24 in this disclosure.Memory 92 may include any fixed or removable magnetic, optical, orelectrical media, such as RAM, ROM, CD-ROM, hard or floppy magneticdisks, EEPROM, or the like. Memory 92 may also include a removablememory portion that may be used to provide memory updates or increasesin memory capacities. A removable memory may also allow patient data tobe easily transferred to another computing device, or to be removedbefore programmer 24 is used to program therapy for another patient.Memory 92 may also store information that controls therapy delivery byIMD 16, such as stimulation parameter values.

Programmer 24 may communicate wirelessly with IMD 16, such as using RFcommunication or proximal inductive interaction. This wirelesscommunication is possible through the use of telemetry module 96, whichmay be coupled to an internal antenna or an external antenna. Anexternal antenna that is coupled to programmer 24 may correspond to theprogramming head that may be placed over heart 12, as described abovewith reference to FIG. 1. Telemetry module 96 may be similar totelemetry module 88 of IMD 16 (FIG. 8).

Telemetry module 96 may also be configured to communicate with anothercomputing device via wireless communication techniques, or directcommunication through a wired connection. Examples of local wirelesscommunication techniques that may be employed to facilitatecommunication between programmer 24 and another computing device includeRF communication according to the 802.11 or Bluetooth® specificationsets, infrared communication, e.g., according to the IrDA standard, orother standard or proprietary telemetry protocols. In this manner, otherexternal devices may be capable of communicating with programmer 24without needing to establish a secure wireless connection. An additionalcomputing device in communication with programmer 24 may be a networkeddevice such as a server capable of processing information retrieved fromIMD 16.

In some examples, processor 90 of programmer 24 and/or one or moreprocessors of one or more networked computers may perform all or aportion of the techniques described in this disclosure with respect toprocessor 80 and IMD 16. For example, processor 90 or another processormay receive one or more signals from electrical sensing module 86, orinformation regarding sensed parameters from IMD 16 via telemetry module96. In some examples, processor 90 may process or analyze sensedsignals, as described in this disclosure with respect to IMD 16 andprocessor 80.

FIG. 10 is a flowchart illustrating techniques for implanting animplantable medical device within a patient. The techniques of FIG. 10are described with respect to IMD 16A, but are also applicable to otherIMDs, such as deployment of leads associated with IMD 16B. First,assembly 180, which includes leadless IMD 16A and catheter 200, ispositioned to a location within the patient, such as right ventricle 28or a vasculature of the patient (502). Next, IMD 16A is deployed fromcatheter 200 to the location within the patient, such as right ventricle28 or a vasculature of the patient (504). For example, the clinician maypush on plunger 212 to deploy IMD 16A.

The clinician evaluates whether IMD 16A is adequately fixated andpositioned within the patient (506). For example, the clinician may usefluoroscopy to evaluate whether IMD 16A is adequately fixated andpositioned within the patient. If the clinician determines IMD 16A isinadequately positioned within the patient, the clinician operatescatheter 200 to recapture IMD 16A by pulling on tether 220 (508). Then,the clinician either repositions distal end of catheter 200 or replacesIMD 16A with another IMD better suited for the implantation location(510). Then step 502 (see above) is repeated.

Once the clinician determines IMD 16A is adequately fixated within thepatient (506), the clinician operates catheter 200 to fully release IMD16A within the patient, e.g., by cutting tether 220 (512). Then, theclinician withdraws catheter 200, leaving IMD 16A secured within thepatient (514).

Various examples of the disclosure have been described. These and otherexamples are within the scope of the following claims.

The invention claimed is:
 1. An implantable medical device comprising:at least one housing; a plurality of electrodes; pacing circuitry withinthe at least one housing, the pacing circuitry configured to delivercardiac pacing via the electrodes; a power source with the at least onehousing, the power source configured to power the pacing circuitry; afixation assembly comprising: a fixation assembly housing fixedlyconnected to the at least one housing proximate a distal end of the atleast one housing; and a fixation element comprising a proximal basewithin the fixation assembly housing and a set of fixation tinesextending distally from the proximal base and out of the fixationassembly housing, wherein each fixation tine in the set comprises ashape memory alloy material and is deployable by bending from aspring-loaded position in which a distal end of the fixation tine pointsaway from the at least one housing to a hooked position in which thefixation tine bends back towards the at least one housing, wherein theset of fixation tines is configured to secure the implantable medicaldevice to patient tissue when deployed while distal ends of the fixationtines are positioned adjacent to the patient tissue.
 2. The implantablemedical device of claim 1, wherein the distal ends of the fixation tinesare proximal to the proximal base when the set of fixation tines are inthe hooked position.
 3. The implantable medical device of claim 1,wherein: the set of fixation tines are in a circular arrangement aboutthe proximal base with proximal ends of the fixation tines secured tothe proximal base; the set of fixation tines are positionedsubstantially equidistant from each other in the circular arrangement;and the set of fixation tines are oriented radially outwardly relativeto the circular arrangement in the hooked position.
 4. The implantablemedical device of claim 3, wherein the set of fixation tines andproximal base are formed as a unitary component from a piece of theshape memory alloy material.
 5. The implantable medical device of claim3, wherein: one electrode of the plurality of electrodes is locatedwithin the circular arrangement; and the implantable medical device isconfigured such that the one electrode contacts the patient tissue whenthe implantable medical device is secured to the patient tissue by theset of fixation tines.
 6. The implantable medical device of claim 5,wherein the set of fixation tines apply a force on the patient tissue ina proximal direction to press the one electrode against the patienttissue, further comprising at least one conductor connected to the oneelectrode, the conductor extending from the at least one housing andthrough the fixation assembly housing.
 7. The implantable medical deviceof claim 5, further comprising at least one conductor configured tocouple the one electrode to the pacing circuitry, wherein the at leastone conductor extends from the at least one housing, through thefixation assembly, and to the electrode, and wherein the fixationassembly housing electrically isolates the set of fixation tines fromthe at least one conductor.
 8. The implantable medical device of claim1, wherein the implantable medical device comprises a leadlesspacemaker.
 9. The implantable medical device of claim 1, wherein the setof fixation tines consists of four fixation tines.
 10. The implantablemedical device of claim 1, wherein the fixation assembly housing isconfigured to electrically isolate the fixation element from the atleast one housing.
 11. The implantable medical device of claim 1,wherein the fixation assembly housing includes a header body and aheader cap configured to securely attach to each other whileencompassing the proximal base of the fixation element near the distalend of the at least one housing such that the fixation element iselectrically isolated from the at least one housing by the fixationassembly housing.
 12. A kit comprising: a catheter forming a lumen thatincludes an aperture to the lumen that is adjacent to the distal end ofthe catheter; and an implantable medical device that includes: at leastone housing; a plurality of electrodes; pacing circuitry within the atleast one housing, the pacing circuitry configured to deliver cardiacpacing via the electrodes; a power source with the at least one housing,the power source configured to power the pacing circuitry; a fixationassembly comprising: a fixation assembly housing fixedly connected tothe at least one housing proximate a distal end of the at least onehousing; and a fixation element comprising a proximal base within thefixation assembly housing and a set of fixation tines extending distallyfrom the proximal base and out of the fixation assembly housing, whereineach fixation tine in the set comprises a shape memory alloy materialand is deployable by bending from a spring-loaded position in which adistal end of the fixation tine points away from the at least onehousing to a hooked position in which the fixation tine bends backtowards the at least one housing, wherein the set of fixation tines isconfigured to secure the implantable medical device to patient tissuewhen deployed while distal ends of the fixation tines are positionedadjacent to the patient tissue, wherein the lumen of the catheter issized to receive the implantable medical device and hold the set offixation tines in the spring-loaded position, and wherein the set offixation tines are configured to be deployed by pushing the implantablemedical device towards the distal end of the catheter until the distalends of the set of fixation tines extend out of the aperture.
 13. Thekit of claim 12, wherein the set of fixation tines are configured togenerate a deployment force during deployment that pulls the attachedimplantable medical device at least partially out of the lumen via theaperture when the distal ends of the set of fixation tines penetrate thecardiac tissue.
 14. The kit of claim 12, wherein the implantable medicaldevice comprises a leadless pacemaker.
 15. The kit of claim 12, whereinthe set of fixation tines consists of four fixation tines.
 16. The kitof claim 12, wherein the fixation assembly housing includes a headerbody and a header cap configured to securely attached to each otherwhile encompassing the proximal base of the fixation element near thedistal end of the at least one housing such that the fixation element iselectrically isolated from the at least one housing by the fixationassembly housing.
 17. The kit of claim 12, wherein the set of fixationtines and proximal base are formed as a unitary component from a pieceof the shape memory alloy material.
 18. The kit of claim 12, wherein:the set of fixation tines are in a circular arrangement about theproximal base with proximal ends of the fixation tines secured to theproximal base; the set of fixation tines are positioned substantiallyequidistant from each other in the circular arrangement; and the set offixation tines are oriented radially outwardly relative to the circulararrangement in the hooked position.
 19. The kit of claim 18, wherein:one electrode of the plurality of electrodes is located within thecircular arrangement; and the implantable medical device is configuredsuch that the one electrode contacts the patient tissue when theimplantable medical device is secured to the patient tissue by the setof fixation tines.
 20. An implantable medical device comprising: atleast one housing; a plurality of electrodes; pacing circuitry withinthe at least one housing, the pacing circuitry configured to delivercardiac pacing via the electrodes; a power source with the at least onehousing, the power source configured to power the pacing circuitry; afixation assembly comprising: a fixation assembly housing fixedlyconnected to the at least one housing proximate a distal end of the atleast one housing; and a fixation element comprising a proximal basewithin the fixation assembly housing and a set of fixation tinesextending distally from the proximal base and out of the fixationassembly housing, wherein each fixation tine in the set comprises ashape memory alloy material and is deployable by bending from aspring-loaded position in which a distal end of the fixation tine pointsaway from the at least one housing to a hooked position in which thefixation tine bends back towards the at least one housing, wherein theset of fixation tines are in a circular arrangement about the proximalbase with proximal ends of the fixation tines secured to the proximalbase, the set of fixation tines are positioned substantially equidistantfrom each other in the circular arrangement, and the set of fixationtines are oriented radially outwardly relative to the circulararrangement in the hooked position, wherein the set of fixation tines isconfigured to secure the implantable medical device to patient tissuewhen deployed while distal ends of the fixation tines are positionedadjacent to the patient tissue, wherein one electrode of the pluralityof electrodes is located within the circular arrangement and theimplantable medical device is configured such that the one electrodecontacts the patient tissue when the implantable medical device issecured to the patient tissue by the set of fixation tines, wherein theimplantable medical device further comprises at least one conductorconfigured to couple the one electrode to the pacing circuitry, whereinthe at least one conductor extends from the at least one housing,through the fixation assembly, and to the electrode, and wherein thefixation assembly housing electrically isolates the set of fixationtines from the at least one conductor.